1. Field of the Invention
The present invention relates to an information recording disc playback apparatus.
2. Description of the Prior Art
An optical disc playback apparatus has an optical pickup for applying a converged laser beam to recording tracks on an information recording surface of an optical information recording disc and detecting changes in the intensity of light reflected from the information recording surface for reading the recorded information. For reading the recorded information accurately, it is necessary that the laser beam be converged on the recording tracks highly accurately at all times regardless of surface fluctuations resulting from the warpage or other surface defects of the information recording disc. To meet this requirement, the optical pickup includes a servo drive mechanism for moving an objective lens along its optical axis (focusing servo) to converge the laser beam. However, some information recording discs may have surface fluctuations which are too large for the servo drive mechanism to control the objective lens to compensate for such surface fluctuations. Therefore, the servo drive mechanism is associated with a height adjusting mechanism for adjusting the height of the optical pickup along the optical axis at a high speed for a stroke greater than the servo stroke of the servo drive mechanism. The height adjusting mechanism incorporated in conventional optical information recording disc playback apparatus is complex in structure, takes up a large space, with the result that the information recording disc apparatus are costly to manufacture and large in size.